Epic Games recently decided to make one of its headline iOS game app Infinity Blade free. Not huge news, that, since it previously only cost $0.99. But as the Wall Street Journal noted, the decision rocketed the game "hundreds" of places up the app download charts.

And this was no one-off, boosted by some promotional help from Apple. No, gaming the app stores by selling your app for free is one heck of a trick.

Consider Readdle, maker of business apps, such as the well-known Scanner Pro, which turns an iPad or iPhone into a mobile document scanner or even a fax machine. During last week's Mobile World Congress the company tried a trick with its PDF Expert app—not exactly the sexiest sounding toy—and dropped its iPhone edition price from $9.99 to zero.

The results, shared with Fast Company by the company's marketing manager Denys Zhadanov, were staggering. From an approximate daily download rate of about 200 global installs a day, the download rate soared to a total of 1.3 million on the first two days it was free. The day before the app went on sale, it sold just 34 copies in the U.S. and just three in Germany, to name just two countries where it's sold. But during the three day free promotion Readdle managed to "sell" over 80,000 copies in the U.S. and 143,000 in Germany. From being in the top 50 of business category apps, the app rocketed to the number one spot in overall free apps—not just the business app category—in Italy, Spain, France, Canada, among other countries. For context, there are now over 700,000 iOS apps in total.

The apps aren't ad-supported, so you could argue that these millions of downloads are effectively a loss to Readdle. Zhadanov doesn't agree, and says instead that the free sales create a halo effect: "It's good enough to have more users. It's viral—the more people who use it the more people will know about it." The company has tried something similar with one of its other apps before, and Zhadanov noted that the short promotion resulted in a lasting doubling of sales rates, even once the app had returned to being a paid-for product. This is thanks to the fact that while free the app zoomed up the App Store popularity ranks, and retained this higher visibility even after it was again priced at $9.99.

Achieving this effect is not simply a question of dropping the price, however, and the move was accompanied by some PR efforts by Zhadanov. It just "won't happen automatically," he explained, noting that his PR efforts resulted in the app being written about on blogs and tweeted, and that helped kick off sales. Still, you can be sure the price didn't hurt.

Recent stats in the Wall Street Journal suggest that global revenue from apps will rise 62% this year to over $25 billion. Playing the app store is very much part of the business for app developers, and there have even been cases such as Chinese developer Qihoo where Apple pulled every app from the app store over alleged manipulation of the app store systems to artificially boost the app popularity ranking, and thus increase its visibility in the app store among the throng of competing apps.